Welcome to ChemZipper !!!: VALENCE BOND THEORY (VBT):

Search This Blog

VALENCE BOND THEORY (VBT):

Valence bond theory explains the bonding in co-ordination compounds. VBT proposed by Pauling and The main postulates of valence bond theory are: 
 (1) The central metal ion makes available a number of empty orbitals for accommodating electrons donated by the ligands. The number of empty orbitals is equal to the coordination number of the metal ion for the particular complex.


COORDINATION NUMBER
(2) These empty atomic orbitals (s, p or d) of the metal ion hybridize to form hybrid orbitals with definite directional properties and give a specific geometry. These hybrid orbitals now axially overlap with the filled orbital of ligand orbitals to form strong coordinate bonds and complex obtained a specific geometry.


HYBRIDASATION AND GEOMETRY
(3) The d-orbitals involved in the hybridization may be either inner (n–1) d-orbitals or outer ndorbitals. The complexes formed in these two ways are referred to as low spin and high spin complexes, respectively.
(4)  Each ligand contains a lone pair of electrons.
(5) A covalent-bond is formed by the overlap of a vacant hybridized metal orbital and a filled orbital of the ligand. The bond is also sometimes called as a coordinate bond.
(6) If the complex contains unpaired electrons, it is paramagnetic in nature, while if it does not contain unpaired electrons, it is diamagnetic in nature.
(7) The number of unpaired electrons in the complex points out the geometry of the complex and vice-versa. In practice, the number of unpaired electrons in a complex is found from magnetic moment measurements as given as
Magnetic moment  (BM)
0
1.73
2.83
3.87
4.90
5.92
Number of unpaired electron
0
1
2
3
4
5
Thus the knowledge of the magnetic moment can be of great help in ascertaining the type of complex.
(8) Strong field ligands affects electronic configuration of central metal my making unpaired electron to pair up, while weak field ligands does not affect electronic configuration of central metal atom i.e. they does not make unpaired electrons to pair up.


SPECTROCHEMICAL SERIES:
Under the influence of a strong ligand, the electrons can be forced to pair up against the Hund’s rule of maximum multiplicity.
Important Note:
Above Statement is valid for first transition series however for second and third transition series unpaired electrons, pair up irrespective of nature of ligands provide pairing of electrons is allowed.

LIMITATION OF VALENCE BOND THEORY

1 comment:

Top Search Topics